The present invention relates generally to afocal optical systems and, more particularly, to methods and apparatus for accurately measuring the magnification of afocal optical systems.
An afocal optical system accepts an input beam of collimated light and creates an output beam that is also collimated. Examples include binoculars, spyglasses, rifle scopes and telescopes. An afocal system does not, by itself, form a final image, and by definition, does not have a finite focal length. However, a comparable first order parameter for such a system is its (afocal) magnification. This is essential to know when combining an afocal optical system with other imaging elements.
Afocal optical instruments, such as binoculars and telescopes are common devices for making a distant object appear larger. It is also quite common to compute the afocal magnifying power of such instruments without independent experimental measurement. The magnification of a simple afocal system can be theoretically computed with knowledge of the individual components and the design of the system, by either using the ratio of the focal lengths of optical components or the ratio of the angles of the incoming and outgoing beams. This approach is consistent with U.S. Pat. No. 4,678,899 to Baba, et al, which discusses a class of variable magnification afocal lens systems where the magnification is changed by moving optical components with respect to each other. The resulting (afocal) magnification can be computed by knowing the locations of the components. However, no independent test is described to confirm it. In addition, if an afocal system is not perfectly aligned, a small amount of beam convergence or divergence may remain in the system. For many applications this slight departure from true afocal performance is not a problem. This slight convergence or divergence of the beam is commonly referred to as xe2x80x9cresidual powerxe2x80x9d, or simply xe2x80x9cpowerxe2x80x9d.
Independent measurement techniques for measuring afocal magnification are known for telescopes and binoculars based on visually comparing the angular subtense of an object, with and without the aid of the binoculars (or a telescope). No special instrumentation is used for this measurement, and accuracy is limited to a few percent with such methods.
More accuracy is attainable by measuring the change in the angle of incoming and outgoing beams, using optical alignment telescopes. Such methods, described hereinafter with reference to FIGS. 1 through 3, may be able to achieve 0.1% accuracy, but are unable to consistently provide the 20 to 50 parts per million accuracy (0.002% to 0.005%) required of certain very high performance afocal systems.
It is therefore an object of the present invention to provide a method for accurately measuring magnification of an afocal optical system.
It is a further object of the present invention to provide a method that allows for simultaneous, closely timed sequential, and/or iterative measurement of wavefront error, power, and magnification of an afocal optical system.
Another object of the present invention to is provide a method that has the ability to adjust or trim either or both the magnification and the power of an afocal system to great accuracy, reducing or eliminating the risk that a system will incur a residual power or wavefront error when adjusting magnification, or vice-versa.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by providing a method for measuring magnification of an afocal optical system comprising the steps of directing a collimated light beam through the afocal optical system, intercepting the collimated beam exiting the afocal optical system with an optical beam splitting device such as a prism, generating two return beams at a first angle therebetween with the optical beam splitting device, passing the two return beams through the afocal optical system, observing an interference pattern created by the two return beams after exiting the afocal optical system, measuring a spacing between at least two fringes of the interference pattern, determining an angle between the two return beams exiting the afocal optical system using the spacing of the at least two fringes of the interference pattern, and comparing the angle between the two return beams exiting the afocal optical system to an angle between the two return beams immediately exiting the prism to thereby measure the magnification of the afocal optical system.
The method of the present invention allows for greater accuracy in the measurement of the magnification of an afocal system. The method is especially useful for making or testing multiple afocal units. A specific example is described for a metrology station capable of unit-to-unit magnification control of better than xc2x130 parts/million.